please review my system design

73 Posts
Jun 28, 2008 11:50 am
please review my system design

I'm ready to buy a couple more panels and a new combiner box, breaker box, charge controller and breakers. Here's what I have designed and would like a 2nd pair of eyes to look at my calcs to see if I'm in good shape.

PV array = (3) PowerUp BSP 20. I currently have (1) and am adding (2) more to the system

BSP 20 specifications
Maximum Power (Pmax) 20W
Voltage at Pmax (Vmp) 17.3V
Current at Pmax (Imp) 1.20A
Short-circuit current (Isc) 1.30A
Open-circuit voltage (Voc) 21.7V

Combiner Box
Midnite Solar MNPV6 Combiner Box

Breaker Box
Square D QO24L70RB Square D QO Load Center

Charge Controller
Xantrex C35 35A, 12/24V PWM Charge Controller

These panels come with 15.5’ of #18 AWG cable pre-attached, so I will be using it to go to the combiner box box. It seems sized correctly for the Isc of 1.3 x 156% = 2.028amps per panel. OR do I need something other than #18awg? I cannot find #18awg in the NEC wire table for 12v systems. I don’t plan on using the entire 15’-6” of cable and will probably end up cutting it down to about 10’-0” in length to get to the combiner box.

I am going to wire them individully into a Midnite Solar MNPV6 Combiner Box and then send the “paralled” combined output to the charge controller. This will give me 60watts at 12vdc nominal. Each panal will go to a 10amp breaker? Again, I cannot find #18awg in the NEC wire table, so I’m assuming that #18awg cannot carry more than 10amps?

The combined wire I will use from the Midnite Solar MNPV6 Combiner Box will be rated for the total Isc of 1.3amps x 3 x 156% = 6.084amps. the wire run length from the combiner box to the breaker panel before the charge controller will be approx  10’-0”. So I will use #8awg for the run from the combiner box to the breaker box. I will use a 40amp breaker in the breaker box before the charge controller.

The last run of wire is from the breaker box to the charge controller. I’ll upgrade my charge controller from a Sunwise Sunguard to Xantrex C35 35A, 12/24V PWM Charge Controller. The wire from the breaker box to the charge controller will only need to go about 6’-0” at the most so I will again use #8awg from the breaker box to the charge contoller. Allowing for expansion in the future.

My plan is to eventually add a couple more BSP20 panels, so I end up with 5 panels. The #8awg should handle the upgrade to (5) panels in the future as well as the rest of the system.

Thoughts?  Comments? Suggestions? Please...

Thanks so much for all the great info I have learned so far through this forum.

James
 
Jun 28, 2008 01:11 pm
Re: please review my system design

It would all seem to be good, but of course, I am not going to be your electrical inspector, so...

It takes a set of rare conditions for a PV module to exceed its Isc ratings by 156% but if it does, as I am sure you already know, we want to "catch" all of that power too, but if your concerned about the amp ratings of that 18 gauge wire then use a breaker just a little bigger than the 2.028, say a 4 amp breaker.

One concern I have, but not so much for your system, is that the Midnight Solar people do not give a total amp rating of the MNPV6 panel. Maybe I just didn't see it.

Again, #8 THHN is rated for 40 amps but that doesn't mean you have to use a 40 amp breaker. You could use a 30 amp breaker. Just cant go higher than 40 of course, but I assume you know that.
Don't forget the EGC (Equipment Grounding Conductor).
 
73 Posts
Jun 29, 2008 07:41 am
Re: please review my system design

Thanks much for looking at my system specs Thomas. By EGC I assume you mean tying in all the equipment to a continuous wire and then sending that wire to a grounding rod right?

James
 
Jun 29, 2008 02:34 pm
Re: please review my system design

Correct. PV module frames, PV array mounts, metal boxes, etc., etc.. If metallic conduit is used to a breaker panel with concentric knockouts for example, then a grounding bushing should be used. If all of the PV system is in a "power shed" it should have its own ground rod as well a third EGC going to any other building it is supplying power to and it should have its own ground rod. One of the biggest reasons for all of this redundant grounding is to insure that if continuity is lost on the EGC somewhere along the way there is always continuity to Earth for an EGC. In my area this is taken very seriously. Inspectors want to see one of two things, ether two ground rods 6 foot apart, or one ground rod certified by a third party. We just drive two.
I can just imagine some guy thinking he was going to start up a business certifying ground rods, only to find that most folks would just drive in two. 
« Last Edit: Jun 29, 2008 02:58 pm by Thomas Allen Schmidt »
 

Disclaimer and Disclosure

The Alternative Energy Store, Inc reserves the right, within its sole discretion, to refuse or delete any posting or portion thereof, or terminate or block the access to this forum.

The opinions and statements posted on this forum are the opinions and statements of the person posting same, and do not constitute the opinion or act of the Alternative Energy Store, Inc (AltE). The Alternative Energy Store, Inc does not endorse or subscribe to any particular posting. No posting shall be construed as the act or opinion of the Alternative Energy Store, Inc.

Click here for BBB Business Review

McAfee SECURE sites help keep you safe from identity theft, credit card fraud, spyware, spam, viruses and online scams
Desktop Website | Mobile Website

Share

Click on an icon to share! If you don't see the method you want, hover over the orange "+".

Feedback

What can we do to help you?

Please enter a summary
Sorry, the copyright must be in the template.
Please notify this forum's administrator that this site is using an ILLEGAL copy of SMF!
Copyright removed!!